Method of manufacturing a liquid-crystal display device

ABSTRACT

A method of forming a liquid-crystal display device having a display cell comprises forming retardation foils on substrattes using polymerized or vitrified liquid-crystal material wherein the liquid-crystal molecules of the polymerized or vitrified liquid-crystal material have a tilt angle with respect to a plane parallel to the substrates and so that the retardation foils have substantially complementary indicatrices and so that each one of the retardation foils brings about the compensation of approximately half the display cell in the driven state.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 08/857,756 filed on May 15, 1997 in the name of Peter Van De Witteet al., the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a liquid-crystal display device having adisplay cell which comprises a layer of a nematic, liquid-crystalmaterial between two substantially parallel substrates, which displaycell is further provided with polarizers. The invention further relatesto a compensator layer which can be used, for example, in liquid-crystaldisplay devices, and to a method of manufacturing a retardation foil.

2. Description of the Related Art

Liquid-crystal display devices are generally used, for example, inmonitors, TV applications and, for example, display devices in motorcarsand for measuring instruments. The retardation foils can also be used inelectro-optical modulators, for example, in welding goggles and inpassive optical elements, such as microscopes and optical systems foroptical data processing.

A display device of the type mentioned in the opening paragraph is knownfrom U.S. Pat. No. 5,210,630. In said display device, a compensator foilconsisting of an optically anisotropic layer of a cholestericallyordered polymeric material is used to counteract discoloration in atwisted nematic display device and to attain a high contrast. Thepolymeric material is ordered in such a manner that a molecular helixcan be distinguished, the axis of the helix being directed at rightangles to the layer.

However, display devices comprising such a compensator still have agreat angle-dependence; this is to be understood to mean that thecontrast is governed to a substantial degree by the angle or directionfrom which the display device is viewed.

One of the objects of the invention is to provide a display device ofthe type mentioned in the opening paragraph, in which theangle-dependence is reduced considerably. A further object of theinvention is to provide a compensator layer which can be used, interalia, in such display devices.

Therefore, a display device in accordance with the invention ischaracterized in that the display cell comprises at least tworetardation foils which predominantly contain polymerized or vitrifiedliquid-crystalline material, the liquid-crystal molecules in thepolymerized liquid-crystalline material exhibiting a tilt angle relativeto the substrates, and the average directions of orientation of theliquid-crystal molecules in the polymerized or vitrifiedliquid-crystalline material of each of the retardation foils making anangle with each other which ranges between 60 and 120 degrees, viewed atright angles to the substrates.

The polymerized, liquid-crystalline material may be partly polymerized,but, preferably, it is polymerized substantially completely.

In this context, the direction of orientation of a liquid-crystalmolecule is to be understood to mean the perpendicular projection on asubstrate of the director of the liquid-crystal molecule. A retardationfoil is to be understood to mean a birefringent foil or layer or a foilor layer having an optically compensating or delaying effect (anoptically anisotropic layer).

The invention is inter alia based on the recognition that as a result ofthe tilt angle of the liquid-crystal molecules in the polymerized orvitrified liquid-crystal material, the retardation of one of theretardation foils compensates, as it were, for the retardation of a partof the liquid-crystal molecules in the display cell in the driven state;the retardation of the other retardation foil compensates, as it were,for the retardation of another part of the liquid-crystal molecules inthe display cell in the driven state.

The invention is further based on the recognition that such layers of apolymerized liquid-crystal material can be manufactured in a simplemanner, for example, by “spin-coating” of nematic liquid-crystalmaterials or by polymerization in the smectic C phase or byvitrification. Dependent upon the manner in which they have beenmanufactured, the liquid-crystal molecules in the polymerizedliquid-crystalline material exhibit a tilt angle relative to thesubstrates, which varies (for example by using surface-active materials)or which is practically constant. This can be determined by means ofconoscopy or microscopy using polarized light (polarizing microscopy).

A preferred embodiment of a display device in accordance with theinvention is characterized in that the direction of orientation of theliquid-crystal molecules in the polymerized or vitrifiedliquid-crystalline material is substantially constant in at least one ofthe retardation foils.

In a liquid-crystal display device in which the customary voltage isapplied across the liquid-crystal material in the on-state, thedirectors in this material actually still make a small angle with thedirection perpendicular to the substrates. As a result, thebirefringence is different at different viewing angles andnon-symmetrical with respect to a direction perpendicular to the twosubstrates, which explains the angle-dependence of a liquid crystalhaving a nematic structure. This birefringence is caused, as it were, bytwo partial layers of a liquid-crystal material in which the opticalproperties of a partial layer are governed by the average tilt angle inthe relevant partial layer with respect to the substrates and by theaverage direction of orientation; if a sufficiently high voltage isapplied across the liquid-crystal layer, then the average direction oforientation in the partial layer is approximately equal to the directionof orientation, as determined by the orientation layers on thesubstrate, so that the magnitude of the difference between thedirections of orientation of the two partial layers is practically equalto the tilt angle. In accordance with the invention, theangle-dependence can be substantially eliminated by causing the averagedirections of orientation of the liquid-crystal molecules in thepolymerized liquid-crystalline material of the retardation foils tointersect each other at an angle which is practically constant (forexample equal to the twist angle of the display cell). The tilt anglesof the liquid-crystal molecules in the polymerized liquid-crystallinematerial can be set relative to the substrates, such that thecompensator composed of the retardation foils compensates theangle-dependence of the cell substantially completely.

During the manufacture of a retardation foil, the tilt of theliquid-crystal molecules (director profile) can be obtained by using apolymeric (or vitrified) material which is formed from a liquid-crystalmonomer.

In principle, any liquid-crystalline polymeric materials can be used toproduce the material for the retardation foils. However, use ispreferably made of liquid-crystalline polymeric materials which are thereaction product of monomers or of a mixture of monomers comprising areactive group. Such polymeric materials have the advantage that theliquid-crystalline groups can be oriented prior to polymerization.Polymerization causes such an orientation to be frozen as it were. It isnoted that such a mixture may also comprise non-reactiveliquid-crystalline monomers. The reactive monomers preferably comprise aliquid-crystalline group.

For the reactive group use can be made of vinyl ethers, thiolene systemsor epoxy groups. However, use is preferably made of reactive groups inthe form of (meth)acrylate groups. Monomers comprising a(n)(meth)acrylate group proved to be excellently processable. In principle,the monomers can be thermally polymerized. In practice,radical-polymerization under the influence of actinic radiation, inparticular UV light, is the simplest way of polymerizing the monomers.This has the advantage that persons skilled in the art can choose thetemperature at which the mixture should be polymerized themselves. Thechoice of the temperature is often very important as theliquid-crystalline properties of the mixture to be polymerized aregoverned to a substantial degree by the temperature.

Preferably, the mixture to be polymerized also comprises monomers havingtwo or more reactive groups of the above-mentioned type. Duringpolymerization, the presence of such monomers leads to the formation ofa three-dimensional network. This causes the optical properties of theinventive retardation foil to become less sensitive to variations intemperature. In particular for foils which are employed at differenttemperatures, such a small temperature-dependence of the opticalproperties is very favorable.

Liquid-crystalline molecules which can be used within the scope of theinvention correspond to the general formulaA-B-M-(B)-(A)

In this formula, M represents a liquid-crystalline group. Suitable Mgroups are disclosed, inter alia, in U.S. Pat. No. 4,398,803 and WO95/24454. B represents a so-called spacer group. Dependent upon thedesired properties, the monomers used comprise one or two spacer groups.Spacer groups are also known from the above-mentioned Patentpublications. A represents a reactive group of the above-mentioned type.The liquid-crystalline molecules may comprise one or two reactivegroups. As stated above, a part of the liquid-crystalline molecules inthe mixture may be non-reactive. In that case, these molecules do notcomprise A-type groups.

A preferred embodiment of the display device is characterized in thatthe polymerized material comprises liquid-crystalline molecules whichare provided, at one end, with a non-polar group and, at the other end,with a polar group. The presence of this type of liquid-crystallinemolecules causes the liquid-crystalline material of the mixture to bepolymerized to assume the homeotropic phase at a short distance from thesubstrate. As a result, the desired ordering of the tilt in theliquid-crystalline material of the retardation foil takes place almostspontaneously. Consequently, in this case treatments with electricfields to induce said tilt are redundant. This simplifies themanufacture of such foils.

Liquid-crystalline molecules having a polar end and a non-polar endcorrespond to the general formulaR-B-M-Z

where B and M have the above-mentioned meaning. In this case, the spacergroup B serves as the non-polar group of the molecule and Z represents apolar group, such as —CN, —OH, —NO₂, —COOH or —C(O)O—CH₃. R represents afurther substituent.

A further preferred embodiment of the display device is characterized inthat at the end provided with the non-polar group, theliquid-crystalline molecules are covalently bonded to the polymerized orvitrified material. This is achieved if for R use is made of a reactivegroup of the above-mentioned type. By virtue of this measure, theoptical properties of the inventive retardation foil become lesssensitive to variations in temperature. In particular for foils whichare employed at different temperatures, such a smalltemperature-dependence of the optical properties is very favorable.

The tilt may be substantially uniform. Alternatively, during themanufacture of the display device, a pretilt can be induced in one orboth boundary surfaces, for example by means of the method described inU.S. Pat. No. 5,155,610. Dependent upon this pretilt, the opticallyanisotropic layer may exhibit, for example, a “splay deformation”. Theeventual director profile can also be influenced, during themanufacture, by means of electric and/or magnetic fields. This mayresult, for example, in a preferred direction for the directors. Such apreferred direction can alternatively be attained during polymerizationin the smectic C-phase of liquid-crystalline materials.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic, cross-sectional view of a part of aliquid-crystal display device in accordance with the invention,

FIG. 2 shows a part of the device shown in FIG. 1,

FIGS. 3 a-3 d schematically explain the optical behavior of a knowndevice, which is similar to the one shown in FIG. 2, by means ofso-called indicatrices,

FIG. 4 schematically shows the differences between the display device inaccordance with the invention and the display device in accordance withFIGS. 3 a-3 d,

FIGS. 5 and 6 schematically show compensator layers in accordance withthe invention,

FIG. 7 shows iso-contrast curves of a display device in accordance withthe invention, and

FIGS. 8 a-8 g show the structural formulas of a number of materialsused.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 is a schematic, cross-sectional view of a part of aliquid-crystal display device comprising a liquid-crystal cell 1 with atwisted nematic, liquid-crystal material 2 sandwiched between twosubstrates 3, 4, for example, of glass, which are provided withelectrodes 5, 6. The device further comprises two polarizers 7, 8 whosedirections of polarization intersect each other at right angles. Thecell further includes orientation layers (not shown), which orient theliquid-crystal material on the inner surfaces of the substrates, in thisexample, in the direction of the polarization axes of the polarizers, sothat the cell has a twist angle of 90 degrees. In this case, theliquid-crystal material has a positive optical anisotropy and a positivedielectric anisotropy. If a voltage is applied to the electrodes 5, 6,the molecules and hence the directors are oriented in accordance withthe field. Thus, in an ideal case, all molecules extend substantiallyperpendicularly to both substrates (situation 11 in FIG. 2). Inpractice, however, this situation requires too high a voltage; atcustomary voltages, the molecules make a small angle with the normal tothe substrates 3, 4, which corresponds to situation 12 shown in FIG. 2.Consequently, when the cell is viewed from direction 13, the viewerlooks much more in the direction of the molecules, so that light whichis still passed at this voltage, is subject to a substantial and, inaddition, asymmetric angle-dependence. This angle-dependence can beexplained by means of the so-called “optical indicatrix”, i.e. athree-dimensional geometric display of the refractive index for eachdirection in which the vector of the electric-field component of thelight can oscillate. In the case of an optically isotropic material,this optical indicatrix is spherical, in the case of a bi-axial materialit is an ellipsoid and, in the case of uniaxial material it is anellipsoid with axial symmetry. As, in an ideal case, the liquid-crystalmaterial in the driven state is uniaxial across almost its entirethickness (in almost all molecular layers, except for a few molecularlayers near the substrates, the molecules extend at right angles to thesubstrates), situation 11 shown in FIG. 2 can be represented byindicatrix 14 in FIG. 3 a, which is an ellipsoid whose main axis extendstransversely to the liquid-crystal layer, the refractive index n_(z) atright angles to the substrates being larger than the refractive index inthe planes extending parallel to the substrates (n_(x)=n_(y)).

As the liquid is not isotropic, birefringence occurs. It can bedemonstrated that this birefringence can be compensated for by anindicatrix 15 in FIG. 3 b, which is an ellipsoid whose axis extendstransversely to the liquid-crystal layer, the refractive index n_(z) atright angles to the substrates being smaller than the refractive indexin the planes parallel to the substrates (n_(x)=n_(y)).

In practice, however, this situation requires too high a voltage; at thecustomary voltages, the molecules make a small angle with the normal tothe substrates 3, 4, which corresponds to situation 12 in FIG. 2. As aresult, when the cell is viewed from direction 13, a viewer looks muchmore in the direction of the molecules. In this more practicalsituation, the indicatrix 14′ has a main axis which makes a small anglewith the axis transverse to the liquid-crystal layer; indicatrix 14 isslightly tilted, as it were, relative to this axis. In this case, a goodcompensation is attained by a compensator layer 9 having an indicatrix15′, which is obtained by tilting, as it were, indicatrix 15 in the samemanner relative to this axis.

FIG. 4 shows, on the left-hand side, the same situation, i.e. the liquid2 with the associated indicatrix 14′, and the compensator layer 9 withthe associated indicatrix 15′; in this case, the liquid and thecompensator layer are sandwiched between crossed polarizers 7, 8. Asshown on the right-hand side of FIG. 4, the display device in accordancewith the invention comprises, in this example, two retardation foilswhich predominantly contain polymerized liquid-crystalline materialhaving a tilt angle of the liquid-crystal molecules in the polymerizedliquid-crystalline material with respect to the substrates and anaverage direction of orientation of the liquid-crystal molecules in thepolymerized liquid-crystalline material, which directions of orientation(in this example) make an angle of 90 degrees with each other, viewed atright angles to the substrates. In this example, the polymerizedliquid-crystal molecules of the retardation foil 9 ^(a) extend parallelto the direction of polarization of polarizer 8 and exhibit an averagetilt angle of 40 degrees. In this example, the polymerizedliquid-crystal molecules of retardation foil 9 ^(b) extend parallel tothe direction of polarization of polarizer 9 and also exhibit an averagetilt angle of 40 degrees. For a possible explanation of the operatingprinciple, the liquid layer 2 is divided into three parts which eachhave their own indicatrix 16, 17 and 18. Indicatrix 21 of retardationfoil 9 ^(a) now compensates, as it were, indicatrix 18 and a part ofindicatrix 17, while indicatrix 22 of retardation foil 9 ^(b) nowcompensates, as it were, indicatrix 16 and the other part of indicatrix17.

The average tilt angle in the retardation foils 9 ^(a), 9 ^(b) may alsobe different, for example 40 and 50 degrees, respectively, which isachieved, in this example, by causing, for example, the other surface ofretardation foil 9 ^(b) to engage the upper surface of retardation foil9 ^(a).

On the one hand, the average tilt angle in the retardation foils 9 ^(a),9 ^(b) is preferably larger than 10 degrees because smaller angles causethe difference in optical behavior between directions parallel andanti-parallel to the projection of the directors on the substrate to betoo small; on the other hand, this tilt angle should preferably notexceed 70 degrees because, otherwise, the retardation foils acquire toomuch axial symmetry as a function of the viewing angle. In the presentcase, the retardation layer is drawn in one piece, however, the layersmay also be situated on either side of the liquid, or the entire layermay be situated on the other side of the liquid.

The associated iso-contrast curve is shown in FIG. 7. In this Figure, Φrepresents the azimuth angle and θ represents the polar angle betweenthe direction of view and the normal to the substrate.

The retardation foils can be manufactured, for example, by providing twoglass plates (whether or not covered with ITO) with orienting layers,(for example polyimide rubbed in anti-parallel directions so that a hightilt is attained) which glass plates are held at a distance from eachother by means of spacers. Between the glass plates, there is provided asuitable mixture of LC monomers, for example a mixture of 25 wt. % C6M(FIG. 8 a) and 74 wt. % 495 (FIG. 8 b) and a suitable initiator,whereafter this mixture is polymerized by UV radiation at 100° C. underthe influence of a weak electric field.

Another suitable mixture comprises 40 wt. % of a reaction LC material (amixture of 25 wt. % 296 (see FIG. 8 c) and 75 wt. % 716 (see FIG. 8 e))and 60% of a non-reactive cyanobiphenyl mixture. This mixture wasspin-coated onto a glass plate covered with rubbed polyimide andsubsequently polymerized by means of UV radiation in a nitrogenatmosphere. As, on the one hand, the molecules are oriented with a smalltilt angle on the polyimide, and, on the other hand, tend to alignhomeotropically on the surface, an average tilt angle α is obtained(FIG. 5). A similar structure is attained with molecules which assume ahomeotropic alignment on the substrate and a planar alignment on thesurface. This can alternatively be achieved by means of other methods(provision by means of a doctor blade) and substrates (directly ontoglass, onto a suitable synthetic resin such as cellulose triacetate).Another mixture, which did not comprise non-reactive liquid-crystallinematerial so that the strength of the layer was increased, was composedof 25 wt. % 296 (see FIG. 8 c) and 75 wt. % 76 (see FIG. 8 e).

A compensator layer 9 is obtained by joining two such retardation foilshaving different tilt angles, the directions of orientation of themolecules being rotated through approximately 90 degrees with respect toeach other. FIG. 5 shows such a compensator layer comprising retardationfoils having different tilt angles. A substrate may be sandwichedbetween the foils. In this case, the director 23 of the polymerizedliquid-crystal molecules in retardation layer 9 ^(a) extends in theplane of the drawing, whereas the director 23 of the polymerizedliquid-crystal molecules in retardation layer 9 ^(b) extends in a planeat right angles to the plane of the drawing (FIG. 5).

In the case of retardation foils having a substantially constant tiltangle, use can also be made of reactive liquid-crystal molecules as thestarting material, which molecules are brought to the smectic C-phasebetween surfaces which bring about a homeotropic alignment, whereafterthe molecules are polymerized again by means of UV radiation. In thismanner, large tilt angles can be achieved (in the range between 40 and89 degrees). As the eventual setting is temperature-dependent, theeventual angle can be influenced via the temperature setting. By way ofexample, use is made of a mixture comprising 54.5 wt. % C6H (No. 23)(see FIG. 8 f), 44.5 wt. % No. 79 (see FIG. 8 g) and a suitableinitiator. The mixture was sandwiched between two glass plates whichwere provided with a layer of a homeotropically aligning material, forexample a polyimide such as SE 7511L which can be obtained from NissanChemical. The mixture was subsequently cooled from 155° Celsius(isotropic state) to 82° Celsius (smectic state). To attain a uniformalignment, a minor shift of the smectic layers may be advantageous.Subsequently, the reactive molecules were polymerized again by means ofUV radiation. FIG. 6 shows how two such retardation foils having aconstant tilt angle β are combined into a compensator layer 9. Ofcourse, the invention is not limited to the above-mentioned examples.For example, the twist angles of the display cell can be chosen to beunequal to 90 degrees, for example in the range between 60 and 120degrees; in general, the angles between the directions of orientation ofthe retardation foils will also be adapted. Twist angles below 60degrees lead to discoloration and imperfect extinction; twist anglesabove 120 degrees cause the transmission/voltage curve to become sosteep that grey levels can no longer be realized.

It is not necessary that the directions of orientation of the moleculesin the retardation foils extend parallel to the planes of polarizationof the polarizers.

It is not necessary either to combine the retardation foils into asingle compensator; in an alternative embodiment one retardation foil isprovided on the side of one polarizer and the other retardation foil isprovided on the side of the other polarizer. In a further example theretardation foils are provided on the outer surface or the inner surfaceof the cell. In the latter case, they can be applied directly onto thesubstrates or on other layers present in the cell, for example on acolor filter or on a protective coating or top coating. If the hardnessof the retardation foil is sufficient, its small thickness (up toapproximately 0.5 Φm) renders it very suitable for use as a top coating.Besides, more than two retardation foils can be used. As mentionedhereinabove, according to the invention the foils are also obtained byvitrification of liquid-crystal molecules instead of polymerization.

In another embodiment viz. a colour liquid crystal display device havinga colour filter the retardation foil has a patterned structure ofdifferent retardation values (e.g. by varying its thickness) inregistration with elements of the colour filter.

For each separate colour the retardation of the associated part of thefoil is optimized for a wavelength associated with said colour.

Instead of driving by means of electrodes on both supporting plates, asdescribed hereinabove, alternative use is made of thermal addressing oraddressing via plasma (plasma-addressed LCD). In the case of very largetilt angles in the retardation foils, it may even be advantageous inspecific cases to provide the molecular structure with a twist so thatthe direction of the maximum contrast can be varied.

In summary, the invention relates to a liquid-crystal display devicecomprising a display cell and a plurality of retardation foils ofpolymerized or vitrified liquid-crystal material, which retardationfoils have substantially complementary indicatrices so that each one ofthe retardation foils brings about the compensation of approximatelyhalf the display cell in the driven state.

1. A method of manufacturing a retardation foil, characterized in that aliquid-crystalline mixture in the smectic C-phase between twohomeotropically aligning substrates is cured by means of polymerization.2. A method of manufacturing a liquid-crystal display device having adisplay cell comprising: forming a layer of a nematic, liquid-crystalmaterial so as to have a twist angle which lies in a range of 60-120degrees, between two substantially parallel substrates; forming firstand second polarizers so as to have first and second polarizingdirections and arranging the first and second polarizers on at least oneof the substrates; and forming first and second retardation foils in apredetermined relationship with the first and second polarizers, and sothat the first and second retardation foils respectively comprisepolymerized or vitrified liquid-crystalline material comprisingliquid-crystal molecules which are respectively arranged to: a) haveaverage orientations which respectively extend in first and seconddirections which directions are respectively parallel to first andsecond planes that are normal to the substrates, the first and secondplanes being oriented with respect to one another at an angle in a rangeof 60 to 120 degrees; and b) exhibit first and second average tiltangles relative to the substrates.
 3. A method as set forth in claim 2,comprising: forming the first and second retardation foils so that atwist angle of the liquid crystal material lies in one of the ranges of60-<90 and >90-120 and so that the angle with which the first and secondplanes are oriented with respect to one another is essentially the sameas the twist angle of the liquid crystal material.
 4. A method as setforth in claim 2, comprising: forming the retardation foils so that anaverage tilt angle of the first retardation foil is 40 degrees.
 5. Amethod as set forth in claim 2, comprising: forming the retardationsfoils so that an average tilt angle of the second retardation foil is 40degrees.
 6. A method as set forth in claim 2, comprising: forming thefirst and second polarizers so that the first and second polarizingdirections are oriented at right angles to each other.
 7. A method asset forth in claim 2, comprising: forming the first and secondpolarizers so that the first and second polarizing directions arenon-parallel with the first and second planes.
 8. A method as set forthin claim 2, comprising: orienting the liquid-crystal molecules in thepolymerized or vitrified liquid-crystalline material so as to besubstantially constant in at least one of the retardation foils.
 9. Amethod as set forth in claim 2, comprising: varying the tilt angle ofthe liquid-crystal molecules in the polymerized or vitrifiedliquid-crystalline material, in at least one of the retardation foils,in a direction at right angles to the foil.
 10. A method as set forth inclaim 9, comprising: forming the retardation foils so that the averagetilt angle of the liquid-crystal molecules in the polymerized orvitrified liquid-crystalline material is at least 10 degrees.
 11. Amethod as set forth in claim 2, comprising: forming the retardationfoils so that the tilt angle of the liquid-crystal molecules in thepolymerized or vitrified liquid-crystalline material is substantiallyconstant in at least one of the retardation foils.
 12. A method as setforth in claim 2, comprising: forming the retardation foils so that thetilt angle of the liquid-crystal molecules in the polymerized orvitrified liquid-crystalline material is at least 10 degrees and at most70 degrees.
 13. A method as set forth in claim 2, comprising: formingthe retardation foils so that the polymerized or vitrified materialcomprises liquid-crystalline molecules which have, at one end, anon-polar group and, at the other end, a polar group.
 14. A method asset forth in claim 13, wherein, at the end having the non-polar group,the liquid-crystalline molecules are covalently bonded to thepolymerized or vitrified material.
 15. A method as set forth in claim 2,comprising: forming the retardation foils so that the direction oforientation of the liquid-crystal molecules in the polymerized orvitrified liquid-crystalline material is substantially constant in atleast one of the retardation foils.
 16. A method as set forth in claim2, comprising: forming the retardation foils so that the tilt angle ofthe liquid-crystal molecules in the polymerized or vitrifiedliquid-crystalline material varies in at least one of the retardationfoils.
 17. A method as set forth in claim 2, comprising: forming theliquid-crystal molecules in the polymerized or vitrifiedliquid-crystalline material so that the tilt angle is substantiallyconstant in at least one of the retardation foils.
 18. A method as setforth in claim 2, comprising: forming the polymerized or vitrifiedmaterial so that the liquid-crystalline molecules are provided, at oneend, with a non-polar group and, at the other end, with a polar group.19. A method as set forth in claim 2, comprising: forming theliquid-crystalline molecules at the end provided with the non-polargroup so that the liquid-crystalline molecules are covalently bonded tothe polymerized or vitrified material.
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 29. A method of forming a liquid-crystaldisplay device having a display cell comprising: forming retardationfoils on substrates using polymerized or vitrified liquid-crystalmaterial wherein the liquid-crystal molecules of the polymerized orvitrified liquid-crystal material have a tilt angle with respect to aplane parallel to the substrates and so that the retardation foils havesubstantially complementary indicatrices and so that each one of theretardation foils brings about the compensation of approximately halfthe display cell in the driven state.